The invention is an electrical power conversion topology and apparatus for converting and delivering power from a mono-polar DC source to an AC load.
Photovoltaic (PV) cells produce power over a wide voltage range depending on the amount of sunlight and the temperature of the photovoltaic cell. There are National Electric Code and class-of-equipment restrictions that make PV arrays much more cost effective when sized for a maximum of 600 Vdc. In order to source AC power into the electric utility grid, over the expected range of DC voltages, prior art utility-interactive inverters use two power conversion stages.
FIG. 6 shows one common prior art inverter topology. Photovoltaic (PV) array 10 is connected to the inverter at input terminals 11 and 12 across energy storage capacitor 59. Transistors 51, 52, 55 and 56 are connected in a typical full-bridge arrangement. For clarity, anti-parallel diodes across each transistor are not shown. The full bridge is driven by a control circuit to regulate sinusoidal current in phase with the electric utility voltage across output terminals 71 and 72. Current sensor 54 provides feedback to the control circuit. Inductor 53 smoothes the high frequency, pulse width modulated (PWM) waveform created by the switching action of transistors 51, 52, 55 and 56. Transformer 60 steps down the utility voltage at terminals 71 and 72 to present a lower voltage to DC-to-AC converter 50 so that power can be delivered from PV array 10 to electric utility grid 70 under all conditions of temperature and irradiation on PV array 10. Electric utility grid 70 is shown as typical, residential, 120/240 Vac, split-phase configuration with a center earth ground. PV array 10 can be operated grounded or ungrounded.
The inverter topology illustrated in FIG. 6 has a number of limitations. First, all of the power from PV array 10 to electric utility grid 70 must be processed twice, once by DC-to-DC converter 50 and once by transformer 60. Transformer 60, from a loss-inventory perspective, is considered an AC-to-AC converter stage. Power is lost in each of these power conversion stages with a negative impact on overall inverter conversion efficiency. Second, transformer 60 operates at the electric utility line frequency and as such is heavy and expensive.
FIG. 7 shows another prior art inverter topology. Photovoltaic (PV) array 10 is connected to the inverter at input terminals 11 and 12. Energy storage capacitor 81, inductor 82, current sensor 83, transistor 84 and diode 85 are arranged as a typical, non-isolated, voltage boost converter. Capacitor 41 is shared by DC-to-DC converter 80 and DC-to-AC converter 50. Transistors 51, 52, 55 and 56 are connected in a typical full-bridge arrangement. For clarity, anti-parallel diodes across each transistor are not shown. The full bridge is driven by a control circuit to regulate sinusoidal current in phase with the electric utility voltage across output terminals 71 and 72. Current sensor 54 provides feedback to the control circuit. Inductors 53 and 57 smooth the high frequency, pulse width modulated (PWM) waveform created by the switching action of transistors 51, 52, 55 and 56. Electric utility grid 70 is shown as typical, residential, 120/240 Vac, split-phase configuration with a center earth ground. PV array 10 must be operated ungrounded.
The inverter topology illustrated in FIG. 7 has a number of limitations. Again, all of the power from PV array 10 to electric utility grid 70 must be processed twice, once by DC-to-DC converter 80 and once by DC-to-AC converter 50. Power is lost in each of these power conversion stages with a negative impact on overall inverter conversion efficiency. Second, when the inverter is operating, there will be large AC common mode voltages, at the utility grid frequency and at the PWM switching frequency, on PV array 10 with respect to earth. The array becomes a radio transmitter. Also, additional conversion losses are had by charging and discharging the parasitic PV-array-to-earth-ground capacitance. In most U.S. jurisdictions, this inverter topology must be used with an external isolation transformer to meet regulatory code requirements.
Other prior-art inverter use a high frequency, double conversion topology which uses a high-frequency, transformer isolated DC-to-DC, voltage boosting converter first stage and a full-bridge DC-to-AC second stage. This topology significantly reduces the inverter weight and cost, a problem with the FIG. 6 topology and mitigates the problem of AC common mode voltage on the PV array, a problem with the FIG. 7 topology. This approach, however, yields the lowest conversion efficiencies because there are too many semiconductor losses. In terms of this discussion, the DC-to-DC stage used in these topologies is, more precisely, a DC-to-High Frequency AC-to-DC converter. Designs based on these topologies are complex, have high component parts counts and, as such, are less robust.
In all prior art topologies discussed, 100% of the throughput power is processed twice and power is lost in each conversion stage. The invention is an improvement over the prior art because the DC-to-AC conversion for the entire PV power converter can be done with 1½ conversion steps, instead of 2, for systems with grounded PV arrays and with effectively less than 1½ conversion steps for systems with ungrounded PV arrays. This translates to at least 25% less complexity, cost and conversion losses over the prior art.